Nanoscale Res Lett (2009) 4:1021–1028
DOI 10.1007/s11671-009-9351-5
NANO EXPRESS
Cobalt and Nickel Nanopillars on Aluminium Substrates by Direct
Current Electrodeposition Process
A. Santos Æ L. Vojkuvka Æ J. Pallarés Æ
J. Ferré-Borrull Æ L. F. Marsal
Received: 26 March 2009 / Accepted: 14 May 2009 / Published online: 31 May 2009
Ó to the authors 2009
Abstract A fast and cost-effective technique is applied for
fabricating cobalt and nickel nanopillars on aluminium
substrates. By applying an electrochemical process, the
aluminium oxide barrier layer is removed from the pore
bottom tips of nanoporous anodic alumina templates. So,
cobalt and nickel nanopillars are fabricated into these templates by DC electrodeposition. The resulting nanostructure
remains on the aluminium substrate. In this way, this method
could be used to fabricate a wide range of nanostructures
which could be integrated in new nanodevices.
Keywords Nanoporous anodic alumina membranes Transfer mask Metallic nanopillars Electrodeposition
Introduction
The template synthesis of nanostructures has attracted
scientists’ attention in the last years owing to their possible
application in fabricating high-density magnetic storage
memories [1] and nanoelectrodes for electrochemical processes in nanometric range [2]. In addition, this kind of
nanostructures could be integrated in smaller and smaller
devices such as filters [3] or sensors [4]. In terms of
nanostructure fabrication, choosing a suitable template is
one of the most crucial factors in the synthesis process,
A. Santos L. Vojkuvka J. Pallarés J. Ferré-Borrull L. F. Marsal (&)
Departament d’Enginyeria Electrònica, Elèctrica i Automàtica,
Universitat Rovira i Virgili, Avda. Paı̈sos Catalans 26, 43007
Tarragona, Spain
e-mail: [email protected]
because any defect in the template structure could be
transferred to the resulting nanostructure by replication. So
far, several materials have been used as template for synthesizing nanowires or nanotubes. Nanoporous anodic
alumina membranes (NAAMs) have become widely used
for the following reasons: first, in contrast to other membranes as polycarbonate membranes, NAAMs present a
higher pore density and a narrower diameter pore distribution [5]. Secondly, both the pore diameter and their interpore distance are rather controllable, because they can
be adjusted by varying the anodization voltage or changing
the electrolyte [6]. Thirdly, by means of a two-step anodization process [7], we can fabricate NAAMs with a selfordered hexagonally and periodic pore arrangement in a
more inexpensive way than with other methods like electron beam lithography [8]. Recently, electrochemical
deposition from an electrolyte has been used [9, 10], since
it is a fast and well-controlled way of fabricating nanowires
and nanotubes by filling porous templates. Nonetheless, asproduced NAAMS have certain disadvantages to be used as
template when an electrochemical deposition is desirable.
The main disadvantage is that there is an aluminium oxide
(Al2O3) barrier layer between the pore bottom and the
aluminium (Al) substrate. This barrier layer electrically
isolates the metallic aluminium substrate from the inner
side of the pores. For this reason, when an electrodeposition of a metallic or semiconducting material is carried out
by direct current (DC) in an as-produced NAAM, it is
rather unstable and there is no uniform filling of the pores.
Moreover, high electrodeposition potentials are needed for
tunnelling the electrons throughout the oxide barrier layer
of the pore bottom. Other deposition techniques like electroless deposition [11], chemical vapour deposition (CVD)
[12] or sol–gel [13] can avoid this drawback, since the
growth of nanowires or nanotubes does not start at the pore
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tips, but from the pore walls. So far, several methods have
been developed for carrying an electrochemical deposition
using NAAMs as template. The most commonly used are
two. In the first one [9, 10], the nanoporous alumina
membrane must be detached from the aluminium substrate
by the dissolution of the Aluminium in a saturated solution
of cupric chloride and hydrochloric acid (HClCuCl2) [14]
or in a saturated solution of mercury (II) chloride (HgCl2)
[15]. Subsequently, the aluminium oxide barrier layer is
removed from the pore bottoms by a chemical etching
process in a solution of phosphoric acid (H3PO4). Finally,
an electrical contact is sputtered on one side of the freestanding NAAM. The second one is the pulsed electrodeposition (PED) method [16], in which the NAAM remains
on the aluminium substrate. By means of this method,
magnetic nanowire arrays of nickel and cobalt have been
fabricated [16, 17]. Nevertheless, only free-standing
metallic nanowires can be fabricated using this method.
In this work, we present an innovative method for fabricating cobalt (Co) and nickel (Ni) nanopillars (NPs) on
aluminium substrates. In contrast to previous works [16,
17], the metallic nanowires remain on the aluminium
substrate after removing the alumina template. Recently,
we have used a technique, previously developed by ourselves, for dissolving in situ the aluminium oxide barrier
layer on the pore bottom tips of NAAMs [18]. We describe
the experimental procedure to fabricate Co and Ni nanopillars as follows: first, we explain the technique used to
achieve the aluminium oxide barrier layer dissolution.
Secondly, we describe the DC electrochemical deposition
process. Thirdly, we show and discuss the results of the
template synthesis method presented, and finally we present our conclusions.
Experimental
Fabrication of Nanoporous Anodic Alumina Membrane
Hexagonally ordered home-made NAAMs were prepared
using direct anodization of aluminium substrates, which is
described in detail somewhere else [19, 20]. First, commercial aluminium substrates (high-purity aluminium
[99.999%] foils from Goodfellow Cambridge Ltd) were pretreated. The aluminium foils were annealed in nitrogen (N2)
environment at 400 °C for 3 h. In this way, both their
crystalline phase and grain size were homogenized. Subsequently, samples were electropolished in a mixture of ethanol (EtOH) and perchloric acid (HClO4) 4:1 (v:v) to reduce
their surface roughness. Finally, the samples were washed
with deionized water, dried under a draught and stored in a
dry environment to prevent the formation of oxide thin films
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Nanoscale Res Lett (2009) 4:1021–1028
because of environmental humidity. Once the aluminium
foils were pre-treated, the anodization process was carried
out following an innovative electrochemical approach for
dissolving in situ the aluminium oxide barrier layer on the
pore bottom tips of the NAAMs [18]. The two-step hard
anodization (HA) procedure was performed on the aluminium surface using an oxalic acid (H2C2O4) solution (0.3 M)
at 0 °C in order to prevent the oxide film burning by catastrophic electric current flow. The first stage of the anodization process was started under constant voltage at 40 V
for 5 min. So, a protective thin layer about 0.5 lm thick was
formed on the aluminium surface. This layer suppresses
breakdown effects due to high temperature and enables
uniform oxide film growth at high voltage. Subsequently,
the voltage was slowly increased to the HA anodization
voltage (120 V) at a constant rate of 0.8 V s-1. The voltage
was then maintained constant for 20 min in order to achieve
a suitable hexagonal arrangement of the pores. When the
first anodization stage finished, the aluminium oxide film
was removed from the aluminium substrate by wet chemical
etching in a mixture of phosphoric acid (H3PO4) (0.4 M) and
chromic acid (H2Cr2O7) (0.2 M) at 70 °C during the same
time of the first anodization stage (about 30 min). In this
way, we produced a pre-pattern on aluminium surface.
Afterwards, the second stage of the anodization process
consisted of directly applying an anodizing voltage of 120 V
in the same electrolyte in which the first stage was carried
out. The anodization voltage was maintained until the
desired pore depth had been reached (around 10 min). Previous studies have found that the rate of film growth is
nonlinear [19], being approximately between 50 and
70 lm h-1. The third stage of the anodization process is
initiated, applying a stepwise current-limited re-anodization
procedure under a galvanostatic regime in the same electrolyte. In this way, the aluminium oxide barrier layer of the
pore bottom tips of the NAAMs was penetrated. In this step,
the previous value of the current density was halved, and the
sample was re-anodized. Then, the voltage fell until it
reached a quasi-steady value. When this almost steady state
had been reached, the current density was again halved and
the voltage decreased again. So, the thickness of the oxide
barrier layer was reduced several tens of nanometres in each
re-anodization step. By means of consecutive repetitions of
this procedure, the oxide barrier layer was penetrated
without the NAAM detachment from the aluminium substrate. Finally, since the aluminium oxide barrier layer is not
uniform in the whole aluminium-alumina interface; the
electrolyte temperature was increased to 30 °C for 30 min to
uniformly remove the rest of the oxide barrier layer from the
pore bottom. In this way, we made sure that the remains of
the aluminium oxide barrier layer were completely removed
from the pore bottom tips.
Nanoscale Res Lett (2009) 4:1021–1028
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Electrodeposition of Co and Ni Nanopillars
After the anodization process, the samples consisted of
NAAMs with opened pores at the aluminium-alumina
interface. At this point, we were able to carry out a DC
electrodeposition under suitable conditions in order to
fabricate Co and Ni nanopillars on aluminium substrates.
The NAAMs acted as a transfer mask in the resulting
structure. During the DC electrodeposition process, the
upper side of the NAAMs was placed in contact with the
corresponding electrolyte solutions. For fabricating Ni
NPs, we used an aqueous solution containing nickel sulphate hexahydrate (NiSO46H2O) and nickel chloride
hexahydrate (NiCl26H2O) as nickel source and boric acid
(H3BO3) as stabilizer. In order to fabricate Co NPs, the
electrolyte consisted of an aqueous solution of Cobalt
sulphate heptahydrate (CoSO47H2O) as cobalt source and
boric acid (H3BO3) as stabilizer. Both aqueous solutions
were constantly stirred at 150 rpm and heated at 40 °C
during the electrodeposition process in order to maintain a
constant concentration of the electrolyte. The concentration and PH of each solution are shown in Table 1. Prior
to the electrodeposition process, the samples were
immersed in the corresponding electrolyte bath for 5 min
in order to completely wet the porous structure. The DC
electrodeposition was carried out using platinum (Pt) wire
as cathode and applying a constant profile of -5 V for Ni
solution and -3 V for Co solution. For characterizing the
Ni and Co NPs when the DC electrodeposition process
was finished, the samples were immersed in a mixture of
phosphoric acid (H3PO4) (0.4 M) and chromic acid
(H2CrO3) (0.2 M) at 70 °C for 30 min in order to dissolve
the NAAM used as template. Finally, the samples were
washed with deionized water and dried under a draught.
All reagents named above were purchased from Sigma–
Aldrich, and a power supply Keithley Model 2420
SourceMeter was used to carry out the DC electrodeposition process.
Table 1 Characteristics of the electrolyte solutions employed for Ni
and Co electrodeposition
Electrolyte
solution
pH
Compounds
Concentration
(g L-1)
Function
Ni
4.5
NiSO46H2O
300
Nickel source
Co
3
NiCl26H2O
45
Nickel source
H3BO3
45
Stabilizer
CoSO47H2O
H3BO3
400
45
Cobalt source
Stabilizer
Fig. 1 ESEM images of a template before and after the complete
removal of the oxide barrier layer from the pore bottom tips. a Crosssection of an AAO film before the removal of alumina barrier layer; b
Magnified view of the area marked in a with a white rectangle; c Pore
bottom detail on which we can see how the pore bottoms are opened
as a whole after the process described has been carried out (the white
arrowheads indicate the pore bottom tips)
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Nanoscale Res Lett (2009) 4:1021–1028
Characterization
Results and Discussion
The morphology and structure of the Ni and Co nanopillars were characterized by an environmental scanning
electron microscope (ESEM FEI Quanta 600). Elemental
qualitative analysis of prepared Ni and Co nanopillars
was carried out using energy dispersive X-ray spectroscopy (EDXS) coupled with the ESEM equipment. The
crystal phases of Ni NPs were analysed by l-XRD
measurements, which were made using a Bruker-AXS
D8-Discover diffractometer, and the crystal phases of
Co NPs were analysed by conventional XRD measurements, which were made using a Siemens D5000
diffractometer.
Once the anodization process was finished, the templates
(NAAMs on aluminium substrates) were inspected by
ESEM image analysis in order to confirm that the oxide
barrier layer was entirely removed from the pore bottom
tips. Figure 1 shows a set of ESEM images of the templates. As we can see, by the end of the process, the oxide
barrier layer has been completely removed from the pore
bottom tips of the NAAMs. Moreover, we have confirmed
that, during this process, the pore diameters increase
slightly (several tens of nanometres), but this is due to the
time that the sample remains in the electrolyte. ESEM
images confirm that the initial structure (Fig. 1a, b), in
Fig. 2 Typical current vs.
time characteristic of the
electrodeposition process.
a Fabrication of cobalt
nanopillars process carried out
at -3 V; b Fabrication of nickel
nanopillars process carried out
at -5 V
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Nanoscale Res Lett (2009) 4:1021–1028
which the pores are closed at the bottom side of the
NAAM, is opened after the process has been carried out
(Fig. 1c). The pore opening is homogeneous throughout the
aluminium–alumina interface, and the NAAM remains on
the aluminium substrate. In this way, we were able to
fabricate suitable templates for carrying out a DC
electrodeposition.
Figure 2 shows the typical current (I) versus time (t)
characteristics, corresponding to the DC electrodeposition
process of Co and Ni nanopillars. As we can see, there are
four different sections in the current curve both for cobalt
(Fig. 2a) and for nickel (Fig. 2b) electrodeposition, also
observed in previous works [21, 22]. In section 1 (S1), the
current decreases abruptly until it reaches a steady value in
section 2 (S2). Then, section 3 (S3) corresponds to a
noticeable increase in the current curve until a second
steady value is reached, corresponding to section 4 (S4).
These four sections can be related to different stages of the
growth of nanopillars in the pores. This process starts using
as template an NAAM with opened pores at the aluminium–alumina interface (Fig. 3a). In the first section, metal
nucleation centres in the pore bottom side start to grow
(Fig. 3b). The decrease in the current can be explained by
local depletion of the ionic concentration at the pore bottom [22]. The current stabilizes when the ionic diffusion
can compensate for this depletion, and the metallic nanopillars grow filling the pores (section 2) (Fig. 3c). When
the pores are entirely filled with Co and Ni, hemispherical
tips of metal grow over the upper end of each nanopillar
(Fig. 3d), resulting in the increase in current observed in
S3. Finally (S4), a metallic film is formed on the NAAM
surface (Fig. 3e). In order to obtain Co and Ni nanopillars
without structural defects, the electrodeposition process
must be finished at the end of the section 2 and the NAAM
template must be removed (Fig. 3f).
As was commented above, after electrodepositing Ni
and Co NPs into the templates, the samples were posttreated in order to be characterized. First, the nanostructures were inspected by ESEM image analysis. Figure 4
compiles a set of ESEM images of the Co NPs (Fig. 4a)
and Ni NPs (Fig. 4b) in which it can be observed that these
nanopillars remain fixed on the aluminium substrates
(Fig. 4c, d). In addition, as Fig. 4e and f show, they keep
the hexagonal arrangement corresponding to the NAAM
used as template during the electrodeposition process. The
average interpillar distance (about 250 nm) corresponds to
the average interpore distance of the template, which
means that the resulting nanostructure is tough enough to
withstand the post-treatment. Moreover, the average pillar
diameter (about 200 nm) is close to the average pore
diameter of the template, and there are no structural defects
in the resulting nanopillars. As we can see, it is confirmed
that the average height of the nanopillars (around 12 lm)
1025
Fig. 3 Slanted cross-section diagram describing the fabrication
process of the metallic nanopillars. a NAAM template on aluminium
substrate once the re-anodization process has finished; b A thin layer
of metal is deposited at the pore bottom; c Rapid growth of metallic
nanopillars inside the NAAM template; d Total filling with metal of
the NAAM template pores; e Metal film formation on the NAAM
surface; f Resulting array of Co and Ni nanopillars when the process
is stopped at the end of the section 2 (S2) [Fig. 2(a) and (b)] and the
NAAM substrate is removed
correspond to the thickness of the NAAM template. These
facts imply that the electrodeposition process was carried
out under suitable conditions, and the filling of the template
pores was practically total. Secondly, in order to confirm
the chemical elements, the nanostructures were analysed by
EDXS. As Fig. 5 shows, both the samples of Co (Fig. 5a)
and Ni (Fig. 5b) nanopillars were exclusively composed of
aluminium (corresponding to the Al substrate) and the
respective metal (Co or Ni), what means that there was no
chemical contamination post-treatment. The quantitative
results were 19.5% Al and 80.5% Co for cobalt nanopillars
and 27.3% Al and 72.7% Ni for nickel nanopillars. At last,
the crystal phase of cobalt and nickel nanopillars was
revealed by XRD analysis. As Fig. 6 shows, both Co and
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Nanoscale Res Lett (2009) 4:1021–1028
Fig. 4 Set of ESEM images of
the metallic nanopillars
fabricated. a Cross-section of an
array of cobalt nanopillars on
aluminium substrate after the
removal of alumina template;
b Cross-section of an array of
nickel nanopillars on aluminium
substrate after the removal of
the alumina template; c Pore
bottom detail on which we can
see how the Co nanopillars are
fixed on aluminium substrate;
d Pore bottom detail on which
we can see how the Ni
nanopillars are fixed on
aluminium substrate; e Top
view of an array of cobalt
nanopillars; f Top view of an
array of nickel nanopillars
Ni patterns of nanopillar arrays presented high-purity
crystal phases since there were not any diffraction peaks of
their corresponding oxides. The main peaks for Co nanopillars are four and appear at 41.6, 44.5, 47.4 and 62.5°,
which correspond to h100i, h002i, h101i and h102i planes
for a hexagonal crystal lattice, respectively (Fig. 6a). The
main peaks for Ni nanopillars are three and appear at 41.5,
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51.8 and 76.4°, which correspond to h111i, h200i and
h220i planes for a face-centred cubic crystal lattice,
respectively (Fig. 6(b)).
In summary, we have reported a simple and innovative
electrochemical approach to fabricate cobalt and nickel
nanopillar arrays on aluminium substrates. This technique
improves other methods previously proposed, because the
Nanoscale Res Lett (2009) 4:1021–1028
Fig. 5 Elemental qualitative analysis of the samples by energy
dispersive X-ray spectroscopy (EDXS). a Spectrum and weight
percentage (inset) of the elements of the sample corresponding to Co
nanopillars; b Spectrum and weight percentage (inset) of the elements
of the sample corresponding to Ni nanopillars
number of stages in the fabrication process is smaller. For
this reason, it is faster and more cost-effective than previous works. This advantage is due mainly to the fact that
the removal of aluminium oxide from the pore bottom tips
in the NAAM template takes place in the same electrolyte
in which the anodization is carried out. Another main
feature of this process is that the Co and Ni nanopillars
remain on the aluminium substrate after removing the
NAAM template. In addition, the technique presented here
can be applied to NAAMs produced by both the MA and
HA techniques with different acids, which opens a wide
range of nanopillar morphologies. The nanopillar diameter
and weight and the interpillar distance can be established
beforehand by modifying the anodization parameters
(anodization voltage, acid and concentration mainly).
By applying this technique with other methods for fabricating this kind of nanostructures, it is expected that the
present method can be used to produce novel nanostructures such as nanotube arrays. This is a promising technique for future applications and a means for fabricating
new nanodevices. One example of a future application of
the resulting structure presented in this work could be using
the metallic nanopillars as nanoelectrodes for the direct
1027
Fig. 6 XRD patterns of cobalt (a) and nickel (b) nanopillar arrays
deposition of nanoparticles from a gas draught. This
nanostructure would act as an electrostatic precipitator by
applying a high-voltage field.
Acknowledgments This work was supported by the Spanish Ministry of Education and Science (MEC) under grant number TEC200606531 and CONSOLIDER HOPE project CSD2007-00007.
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